A secondary battery has high applicability depending on a product group and excellent electrical characteristics such as high energy density, and thus is commonly used as an electric power source of electric vehicles (EV), hybrid vehicles (HV) and energy storage systems.
A battery tends to gradually degrade in terms of its performance during repeated charging and discharging cycles. Such a degradation of the battery may be confirmed by the gradual decrease in capacity as the charging/discharging cycle of the battery increases.
The degradation of the battery is mainly due to the irreversibility of an electrochemical reaction. That is, when the charging/discharging cycle of the battery increases, the characteristics of a substance which participates in an electrochemical reaction is deteriorated by an aging effect, so the electrochemical reaction is not reversibly carried out during a charging and discharging process.
As well known in the art, the degradation of the battery brings about the decrease of usable electrical energy, thereby decreasing the use time of an instrument or device which is supplied with the energy from the battery and deteriorating output characteristics. For example, in the case a battery of an electrical vehicle is degraded, the amount of distance driven by one charge cycle decreases under the same condition. Therefore, it is necessary to apply a battery-related technology capable of maintaining the performance of an electrical vehicle regardless of the degradation of a battery.
Meanwhile, interest of a smart grid has been increasing recently due to the depletion of fossil fuels. The smart grid is based on a dispersion power management model, and stores dump power in a bulk power storage apparatus to use the remaining power in a power system later. Since a power storage apparatus has characteristics of having to be used for a long period once it is installed, it is important for a battery used in the power storage apparatus to maintain uniform output characteristics for a long period, rather than have high output characteristics, unlike a battery used in an electrical vehicle. However, if a charging and discharging process is repeated under the condition of an appointed voltage range, the battery capacity decreases, thereby failing to maintain constant output characteristics thereof. Accordingly, there is a need for maintaining a constant battery capacity regardless of the degradation of the battery.